This invention relates generally to intermediate frequency (IF) signal amplifiers and is particularly directed to a gain equalized intermediate frequency signal amplifier utilizing a combination of bipolar and field effect transistors.
An intense effort is currently under way to develop a low cost, high performance television system which utilizes satellites in geosynchronous orbit to relay signals from earth-based transmitting stations to small receivers owned or leased by individual television viewers. This system offers the advantages of availability in remote locations as well as increased frequency band capability to permit the transmission of a large number of programs in addition to that available via terrestrial television signal transmission.
The satellite television signals are generally transmitted in the super high frequency (SHF) band because of the favorable propagation characteristics at these frequencies and the relatively small size of components used for processing signals at these frequencies. Each satellite receiver includes a remotely located low noise outdoor unit comprised primarily of a frequency down converter connected to a dish antenna and an indoor unit comprised primarily of a channel selecting portion and an FM demodulating portion. The outdoor unit is typically mounted at the focal point of the parabolically-shaped dish antenna and converts the SHF (.about.12 GHz) input signal to a UHF (.about.1 GHz) intermediate frequency signal where it is amplified in an IF amplifier to a level high enough to overcome the losses in the coaxial cable connecting the outdoor unit with the viewer's indoor unit. The IF frequency band may cover several hundred megahertz, with a required gain of 30 dB or more and a noise figure of 2 dB or better.
With these performance criteria in mind, IF amplifiers may be categorized in terms of four general areas. These areas are briefly outlined in the following paragraphs.
A. Technology level of the manufacturing process: PA0 B. Basic semiconductor material the active device is made from: PA0 C. The number of discrete inductive circuit elements, adjustable or nonadjustable, used for tuning, matching and compensation: PA0 D. Price:
1. Packaged discrete device level with the discrete devices mounted on a printed circuit (PC) board together with various passive devices also in discrete form; PA1 2. Hybrid circuit level utilizing either thick or thin film technology, usually with active elements in chip form bonded to the conductors on the substrate and possibly with some discrete passive chip components added to the circuit; or PA1 3. Monolithic circuit level with all active and passive elements implemented by means of special processing methods on a single chip. PA1 1. Silicon; or PA1 2. Gallium arsenide. PA1 1. None; PA1 2. A Few; or PA1 3. Many. PA1 1. Inexpensive; or PA1 2. Expensive.
While, for example, in the military sector the typical dominating factor is performance, in the consumer electronics sector the typical dominating factor is price and obviously the emphasis is therefore on optimum performance at the lowest possible price.
While IF amplifiers built on the monolithic circuit level are at the present time still in the laboratory stage (Monolithic Circuits Symposium--digest of papers June 1982, IEEE Catalog No. 82CH1784-8), and not readily available, amplifiers made as hybrid circuits are available off the shelf. For example, Watkins-Johnson offers an extensive line of products of this kind. Unfortunately, the price of a hybrid IF amplifier is for TV manufacturers prohibitive, unless the entire receiver, i.e., SHF and IF sections, is made on a common substrate in hybrid form with the hybrid approach justified mainly by the need for the SHF section. The last, and probably the least expensive option, is an IF amplifier implemented on a single sided PC board (conductive foil placed only on one side of the dielectric substrate) and with packaged discrete elements. The choice now is between silicon and gallium arsenide (bipolar and GaAs FET) and includes consideration of criterion "C" above. Since a low cost circuit is also a circuit with a minimum of adjustable elements, the tendency would be toward using only capacitively coupled bipolar stages. In actual practice, however, the inherent parasitic capacitances and inductances whether residing in the active devices or due to the PC board pattern and dielectric material make this approach very difficult, if not impossible.
Furthermore, the reactive parasitic elements of the PC board will be present whether silicon or gallium arsenide devices are used so that discrete GaAs FET devices used as active elements in such a PC board based amplifier do not have a very significant advantage over bipolar transistors. In addition, GaAs FET's have another disadvantage in the high Q of their equivalent input and output circuits which makes it more or less impossible at UHF frequencies to obtain a relatively large gain over the IF band in these devices. The other option--inexpensive bipolar transistors--is not as difficult in terms of conjugate matching and provides a reasonable gain at the low end of the IF band, but may have a gain slope of as great as 6 dB/octave.
The present invention contemplates the economical use of both bipolar and GaAs FET devices. In this way, a capacitively coupled second portion of an IF amplifier implemented by means of inexpensive bipolar transistors is compensated for by means of a first GaAs FET portion designed so that it provides its available gain selectively at the high end of the band thus compensating for the limited gain thereover of the bipolar portion. With the proper distribution of the coupling element values, a reasonably flat gain versus frequency curve is provided.